1. Field of the Invention
This invention relates to squarylium compounds which absorb near infrared rays, and to an optical information recording medium using the squarylium compounds.
2. Discussion of Background
Materials capable of absorbing near infrared rays have various uses, for example, for (i) safe-light filters for photosensitive materials which are sensitive to infrared rays; (ii) infrared filters for controlling the growth of plants; (iii) materials for intercepting the heat rays in sunlight; (iv) infrared filters for protecting eye tissues from infrared rays; and (v) infrared filters for the light receiving elements of semiconductors.
In addition to the above, a recording material of an optical information recording medium is one of the important applications of the above-mentioned near-infrared-ray-absorbing materials.
Conventionally, cyanine dyes such as phthalocyanine dyes; phenanthrene dyes; naphthoquinone dyes; and pyrylium dyes are known as materials capable of absorbing near infrared rays. In Japanese-Laid-Open Patent Applications 55-97033, 58-83344, 58-224793, 58-214162 and 59-24692, optical information recording media are disclosed which comprise the aforementioned dyes as recording materials.
Each of the above-mentioned dyes, however, has its own shortcomings. More specifically, phthalocyanine dyes are disadvantageous in that they have low photosensitivity and high decomposition point, and are not readily vacuum-deposited. In addition, since they have extremely low solubility in organic solvents, they cannot be applied by liquid coating.
Phenanthrene dyes and naphthoquinone dyes are readily vacuum-deposited, but low in reflectance. The low reflectance causes a low contrast between one portion in which information is recorded by application of laser beams and the other portion in which no information is recorded. This will bring about inaccurate reproduction of the recorded information.
Pyrylium dyes and cyanine dyes are soluble in solvents, so that they can be applied by liquid coating. However, these dyes are so poor in light-resistance that they are readily deteriorated when exposed to the light employed for the reproduction of recorded information.